1. Field of the Invention
The present invention relates to an apparatus for optically detecting a displacement of an object (amount of movement, amount of rotation angle, velocity, acceleration), and more particularly to an encoder, a velocity sensor and an acceleration sensor and the like.
2. Related Background Art
An optical-type displacement sensor which irradiates light onto an object to obtain the displacement of the object with great precision such as an optical encoder, a laser doppler velocity sensor and a laser interfero-meter has widely been used in the fields of an NC machine tool, an OA equipment, a robot and a precision manufacturing equipment and the like.
In some such displacement sensors, a 0-order diffraction light and a 1-order diffraction light obtained by diffracting a laser beam with a first diffraction grating are irradiated onto a second diffraction grating which is formed on a scale, and a +1-order reflected diffraction light generating from the 0-order diffraction light generated by reflecting and diffracting 0-order diffraction light with a second diffraction grating and a -1-order reflected diffraction light generated by reflecting and diffracting the 1-order diffraction light with the second diffraction grating are directed to a third diffraction grating on the side of the first diffraction grating, and with the third diffraction grating the +1-order reflected diffraction light and the -1-order reflected light are combined to form an interference pattern, which is photo-electrically converted to produce a sine wave signal representing the displacement of the scale.
In the above displacement sensor, since each of the diffraction lights which form the interference pattern is subjected to diffraction and split operations a plurality of times by conventional diffraction gratings, the intensity of each diffracted light is small and as a result the intensity of the interference light is small. As a result, a problem has arised that an S/N ratio of the sine wave signal produced by the photoelectric conversion is low.
A similar problem has resulted in other displacement sensors in which a diffraction grating is used.